1. Field of the Invention
This invention relates to an improved metal flow system or runner/gate arrangement, for use in the production of pressure castings made from aluminium alloys, such as but not exclusively in a molten or thixotropic state, suitable for use with various forms of pressure casting machines including, but not limited to, existing hot and cold chamber die casting machines.
2. Description of Related Art
An understanding has developed throughout the international pressure casting industry that it is necessary to use large runners to prevent premature freezing of the molten aluminium alloy metal during pressure casting. Within the industry, there are many different design methods which are thought to provide satisfactory castings from aluminium alloys. However, common to these different methods is a reliance on runner systems of large volume relative to casting size and low metal flow velocities through the runners.
To illustrate the large volume runner systems used by current systems in pressure casting of aluminium alloys, it is usual for a foundry having an annual casting production level of 250,000 tonnes of saleable castings to have processed some 450,000 tonnes of alloy, where the weight of sprue/runner metal of alloy is about 200,000 tonnes. In this production, it is usual to use oversized runners, in order to prevent alloy freeze-up, with the result that runner velocities of about 10 m.sec−1 are achieved. Corresponding gate velocities are about 30-45 m.sec−1, with the gate velocity more usually being in the range of 30-35 m.sec−1. Of the aggregate quantity of melt poured, only about 55% results in productive output. As a consequence, there is a need for an excessive inventory of aluminium alloy required to allow for the remaining metal consumed as runner metal to be recycled. There accordingly is a high level of excess energy consumption in heating alloy which, after casting, needs to be recovered and recycled. Also, it is typical for there to be alloy loss at a level of about 3% of the total tonnage poured which, on the indicated level of foundry output, represents a loss of about 13,500 tonnes (at a cost of about AU$30M).
In such production, there are significant costs additional to the high level of aluminium alloy inventory, the loss of alloy and the cost of heating, recovery and recycling runner/gate alloy. At the level of output indicated, there may be five furnaces required for preparation of molten alloy for casting. Such furnaces can cost about AU$15M each, and reducing the number of these furnaces by only one, along with its ancillary equipment, would achieve a substantial saving in capital expenditure. Also, casting die costs can amount to about 15% of overall production cost, and an improvement in die life would provide substantial scope for further savings. Indeed, the overall cost burden is such that it serves to highlight how entrenched is the thinking on established foundry practice on pressure casting of aluminium alloys.
We have found that, by use of the present invention, it is possible and practical to produce high quality pressure castings of aluminium alloys of at least comparable quality to those provided by established foundry practice, but with substantial cost savings. The nature of the cost savings are detailed later herein.